This invention relates generally to integrated circuits. More specifically, this invention relates to a programmable logic device with buffers that may be selectively configured for Schmitt-triggered or threshold-triggered operation.
Buffer circuits are one of the building blocks of a digital system. A typical integrated circuit has many numbers of buffers. A typical buffer, such as an inverter, noninverting buffer, input buffer, or output buffer, detects a level change at its input based on a fixed switching threshold. For an inverting buffer, an input signal above this threshold level, the output will be at a particular logic level (e.g., logic 0). For an input signal below this threshold, the output will be at the other logic level (e.g., logic 1).
Digital systems often operate under noisy conditions. Noisy conditions are usually defined as environments where there are signals switching unrelated to the signal of interest. In some cases, the noise content caused by coupling can be so large that false switching occurs. That is, noise can cause a metastable environment in which a digital low signal appears as a digital high signal, or vice versa. This sensitivity is especially acute for signals making slow transition from 1 to 0 or vice versa. They spend more time near the threshold where sensitivity is more acute.
An approach to eliminating or reducing the effects of noise in a digital system is to employ logic that relies upon a hysteresis detection scheme, instead of a fixed threshold detection scheme. In electronics, hysteresis refers to the property of a device to transition the output based on an asymmetric threshold voltage. The threshold is higher for a 0 to 1 input transition than a 1 to 0 input transition. Schmitt-trigger circuits are particularly useful for providing a smooth reliable output from a circuit that may have some noise on the input. This ability to smooth-out noise is increasingly important for integrated circuits, especially as supply voltage levels are dropping, which generally decreases input noise margins. This is also important for slow rising or falling inputs.
Schmitt trigger circuits generally have a propagation delay that is greater than a similar sized buffer circuit without hysteresis. Therefore, in situations where higher performance (i.e., a faster speed path), it would be advantageous to use a fixed or symmetric threshold-triggered buffer over a Schmitt trigger circuit.
Therefore, it would be highly desirable to improve the noise immunity of selected buffers in a programmable logic device by utilizing Schmitt-trigger technology, while simultaneously exploiting fixed or symmetric threshold-triggered buffers at circuit nodes in programmable logic devices that have certain critical timing requirements.